diff --git a/core/lib/Drupal/Core/StreamWrapper/LocalStream.php b/core/lib/Drupal/Core/StreamWrapper/LocalStream.php
index ae0a435..db51eda 100644
--- a/core/lib/Drupal/Core/StreamWrapper/LocalStream.php
+++ b/core/lib/Drupal/Core/StreamWrapper/LocalStream.php
@@ -132,7 +132,7 @@ protected function getLocalPath($uri = NULL) {
     if (!isset($uri)) {
       $uri = $this->uri;
     }
-    $path = $this->getDirectoryPath() . DIRECTORY_SEPARATOR . $this->getTarget($uri);
+    $path = $this->getDirectoryPath() . '/' . $this->getTarget($uri);
 
     // In PHPUnit tests, the base path for local streams may be a virtual
     // filesystem stream wrapper URI, in which case this local stream acts like
@@ -145,7 +145,7 @@ protected function getLocalPath($uri = NULL) {
     $realpath = realpath($path);
     if (!$realpath) {
       // This file does not yet exist.
-      $realpath = realpath(dirname($path)) . DIRECTORY_SEPARATOR . drupal_basename($path);
+      $realpath = realpath(dirname($path)) . '/' . drupal_basename($path);
     }
     $directory = realpath($this->getDirectoryPath());
     if (!$realpath || !$directory || strpos($realpath, $directory) !== 0) {
@@ -453,7 +453,7 @@ public function mkdir($uri, $mode, $options) {
     if ($recursive) {
       // $this->getLocalPath() fails if $uri has multiple levels of directories
       // that do not yet exist.
-      $localpath = $this->getDirectoryPath() . DIRECTORY_SEPARATOR . $this->getTarget($uri);
+      $localpath = $this->getDirectoryPath() . '/' . $this->getTarget($uri);
     }
     else {
       $localpath = $this->getLocalPath($uri);
diff --git a/core/modules/system/src/Tests/File/SystemStreamTest.php b/core/modules/system/src/Tests/File/SystemStreamTest.php
index 94c85db..4bb8b0e 100644
--- a/core/modules/system/src/Tests/File/SystemStreamTest.php
+++ b/core/modules/system/src/Tests/File/SystemStreamTest.php
@@ -462,10 +462,6 @@ private function runTestOnInstanceMethod(StreamWrapperInterface $instance, $meth
         }
       }
       elseif (is_string($case)) {
-        if ($method == 'realpath') {
-          // realpath() returns strings with OS-specific DIRECTORY_SEPARATOR.
-          $case = str_replace('/', DIRECTORY_SEPARATOR, $case);
-        }
         $this->assertEqual($instance->$method(), $case);
       }
     }